1. Field of the Invention
The present invention relates generally to a gas turbine engine, and more specifically to a turbine rotor blade with tip region sealing and cooling.
2. Description of the Related Art Including Information Disclosed Under 37 CFR 1.97 and 1.98
A gas turbine engine, such as an industrial gas turbine (IGT) engine, includes a turbine with multiple rows or stages or stator vanes that guide a high temperature gas flow through adjacent rotors of rotor blades to produce mechanical power and drive a bypass fan, in the case of an aero engine, or an electric generator, in the case of an IGT. In both cases, the turbine is also used to drive the compressor.
Passing a higher temperature gas flow into the turbine referred to as the turbine inlet temperature can increase the efficiency of the gas turbine engine. The highest temperature gas flow is found in the entrance to the first stage stator vanes and rotor blades, since the rotor blades progressively decrease the gas flow temperature as they removed energy from the gas flow stream. Higher temperature resistance materials can be used for these airfoils to allow for higher turbine inlet temperatures. Also, better cooling can be used for these airfoils to allow for use of the same materials but under higher gas flow temperatures. However, the pressurized cooling air used to cool these airfoils is typically bled off from the compressor, which is compressed by work from the engine in which this work is not used to produce power. Thus, using too much cooling air will also reduce the engine performance.
Especially in an industrial gas turbine (IGT) engine, long life for the turbine airfoils is critical, since these engines operate for very long periods of time. Designers and engine operators hope for a constant run time between engine shutdowns of at least 40,000 hours. Since the engine airfoils are exposed to extreme operating conditions, erosion or corrosion are important features that must be addressed in airfoil design. One hot spot occurring on an airfoil can result in the airfoil losing shape or burning a hole in the surface that can cause hot gas injection or too much cooling air to be discharged.
Blade tip region cooling and sealing is an important region to be addressed by a blade designer. Tip cooling is required to prevent hot spots from occurring that can lead to erosion of the blade tip. Limiting the tip leakage flow is required to improved performance of the turbine as well as to reduce an over-temperature on the tip region that would occur due to high amounts of hot temperature gas flowing through the tip clearance. High temperature turbine blade tip section heat load is a function of blade tip leakage flow. A high leakage flow will induce high heat load onto the blade tip section. Therefore, blade tip section sealing and cooling have to be addressed as a single problem. A typical turbine blade tip will include a squealer tip rail that extends around a perimeter of the airfoil flush with the airfoil wall to form an inner squealer pocket. The main purpose of incorporating a squealer tip in a blade design is to reduce the blade tip leakage and also to provide rubbing capability for the blade against an inner shroud surface of the casing. Allowing for slight rubbing will reduce the gap clearance of the blade tip to zero. FIG. 1 shows a prior art turbine rotor blade with a squealer tip cooling design. In general, film cooling holes are built into the blade along the airfoil pressure side tip section and extend from a leading edge to the trailing edge to provide edge cooling for the blade pressure side squealer tip. In addition, convection cooling holes also built into the tip rail at the inner portion of the squealer pocket provide additional cooling for the squealer tip rail. Secondary hot gas flow migration around the blade tip section is also shown in FIG. 2. A vortex flow pattern is formed on the blade suction side as indicated by the vortex flow swirling in FIG. 2.
FIGS. 3 and 4 show a prior art turbine rotor blade with cooling holes for the blade pressure side and suction side tip rails. The blade tip rail is subject to heating from three exposed sides. Cooling of the pressure side and suction side squealer tip rail by means of a discharge row of film cooling holes along the blade peripheral and at the bottom of the squealer floor therefore becomes insufficient. This is primarily due to the combination of tip rail geometry and the interaction of hot gas secondary mixing The effectiveness induced by the airfoil surface film cooling and the tip section convective cooling holes is very limited.